To measure energy expenditure (EE) and rates of lipolysis, fat oxidation and triglyceride synthesis in Prader-Willi syndrome (PWS) patients and to see if this differs from matched obese controls. To compare adipocyte morphology and lipoprotein lipase activity in PWS patients and controls. We will try to determine whether these variables differ among chromosomal and DNA subtypes of people with PWS. Also, we plan to determine whether there are changes in energy expenditure and fat metabolism in vivo following exercise training in patients.